This invention relates to a method for preparing polyolefins having a broadened or multimodal molecular weight distribution.
Polyolefins having a multimodal molecular weight distribution (MWD), such as polyethylene, can be made into articles by a variety of methods, including, but not limited to, extrusion molding, thermoforming and rotational molding, and have advantages over typical polyolefins lacking a multimodal MWD. Polyolefins having a multimodal MWD process more easily, i.e., they can be processed at a faster throughput rate with lower energy requirements. At the same time, such polymers can exhibit reduced melt flow perturbations and are preferred because of improved properties for applications such as blow molding and/or high strength films. Polymers having a multimodal MWD generally are characterized by having a broad MWD, or more that one MWD peak, as reflected by size exclusion chromatography (SEC) curves.
There are several known methods of producing polyolefins having a multimodal MWD; however, each method has its own disadvantages. For example, polyolefins having a multimodal MWD can be made by employing two distinct and separate catalyst systems in the same reactor, each producing a polyolefin having a different MWD; however, catalyst feed rates are usually difficult to control and the catalysts can have a detrimental effect on each other. Polymer particles produced from a dual, or even multi-, catalyst system frequently are not in uniform size. Thus, segregation of the polymer during storage and transfer can produce non-homogeneous products. Polyolefins having a multimodal MWD also can be made by sequential polymerization in two or more separate reactors or made by blending polymers of different MWD during processing; however, both of these methods increase capital cost and problems discussed earlier regarding polymer segregation can occur.